Blanc, Guillermo A. and Schruba, Andreas and Evans, Neal J., II and Jogee, Shardha and Bolatto, Alberto and Leroy, Adam K. and Song, Mimi and van den Bosch, Remco C. E. and Drory, Niv and Fabricius, Maximilian and Fisher, David and Gebhardt, Karl and Heiderman, Amanda and Marinova, Irina and Vogel, Stuart and Weinzirl, Tim (2013) The VIRUS-P Exploration of Nearby Galaxies (VENGA): The X CO Gradient in NGC 628. Astrophysical Journal, 764 (2). Art. No. 117. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20130315-082008467
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We measure the radial profile of the ^(12)CO(1-0) to H_2 conversion factor (X_(CO)) in NGC 628. The Hα emission from the VENGA integral field spectroscopy is used to map the star formation rate (SFR) surface density (Σ_(SFR)). We estimate the molecular gas surface density (Σ_(H2)) from Σ_(SFR) by inverting the molecular star formation law (SFL), and compare it to the CO intensity to measure X_(CO). We study the impact of systematic uncertainties by changing the slope of the SFL, using different SFR tracers (Hα versus far-UV plus 24 μm), and CO maps from different telescopes (single-dish and interferometers). The observed X_(CO) profile is robust against these systematics, drops by a factor of two from R ~ 7 kpc to the center of the galaxy, and is well fit by a gradient Δlog(X_(CO)) = 0.06 ± 0.02 dex kpc^(–1). We study how changes in X_(CO) follow changes in metallicity, gas density, and ionization parameter. Theoretical models show that the gradient in X_(CO) can be explained by a combination of decreasing metallicity, and decreasing Σ_(H2) with radius. Photoelectric heating from the local UV radiation field appears to contribute to the decrease of X_(CO) in higher density regions. Our results show that galactic environment plays an important role at setting the physical conditions in star-forming regions, in particular the chemistry of carbon in molecular complexes, and the radiative transfer of CO emission. We caution against adopting a single X_(CO) value when large changes in gas surface density or metallicity are present.
|Additional Information:||© 2013 American Astronomical Society. Received 2012 August 16; accepted 2012 December 13; published 2013 January 30. The VENGA Collaboration acknowledges the generous support from the Norman Hackerman Advanced Research Program (NHARP) ARP-03658-0234-2009, G.A.B. acknowledges the support of Sigma Xi, The Scientific Research Society, Grantin-Aid of Research. N.J.E. acknowledges the support of NSF grant AST 1109116. A.D.B. acknowledges partial support from grants NSF AST-0838178, NSF AST-0955836, as well as a Cottrell Scholar award from the Research Corporation for Science Advancement. N.D. acknowledges support from PAPIIT grant IA-100212. We also acknowledge Desika Narayanan for providing the curves presented in Figure 5, and for useful advice regarding the application of his model, and Karin Sandstrom useful discussions and for providing her data for comparison. Support for CARMA construction was derived from the Gordon and Betty Moore Foundation, the Eileen and Kenneth Norris Foundation, the Caltech Associates, the states of California, Illinois, and Maryland, and the NSF. Funding for ongoing CARMA development and operations are supported by NSF and CARMA partner universities. The construction of the Mitchell Spectrograph (formerly VIRUS-P) was possible thanks to the generous support of the Cynthia & George Mitchell Foundation. This research has made use of NASA’s Astrophysics Data System, and of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.|
|Subject Keywords:||galaxies: abundances; galaxies: ISM; galaxies: spiral; ISM: clouds; ISM: molecules|
|Official Citation:||The VIRUS-P Exploration of Nearby Galaxies (VENGA): The X CO Gradient in NGC 628 Guillermo A. Blanc et al. 2013 ApJ 764 117|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||18 Mar 2013 20:58|
|Last Modified:||18 Mar 2013 20:58|
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